Passive infra red detectors are known for providing presence and absence detection to control, for example, lighting. However, in some circumstances current designs of passive infra red detector are ineffective, particularly where the detector is a relatively large distance from the detection field. For example, in large warehouses, the height of the warehouse may be well in excess of ten meters. It is desirable to detect the presence of a worker, for example in a particular stock bay, in order to turn on lighting when the worker is present and to turn off the lighting when the worker exits. In this way, it is not necessary for the lighting to be on in every bay of a warehouse all the time and energy is saved. However, where the bay is high and the detector is mounted in the ceiling or roof of the warehouse the distance to the detector from the floor of the warehouse is very large. Current detector designs are incapable of accurately detecting presence and absence of personnel in such circumstances.